7. PROJECT SUMMARY Transgender women (TW) are disproportionately impacted by HIV and have some of the highest incidence and prevalence rates in the US and around the world. PrEP has tremendous potential to reduce rates of HIV infection among TW; however, there is a striking lack of research on PrEP in this population. Data are urgently needed regarding the unique needs, concerns, HIV risk behavior patterns and health care utilization dynamics that impact PrEP initiation, persistence, and adherence among TW. In addition, more data are needed on adherence behavior and drug concentrations among TW, especially in the context of community concerns about drug-drug interactions (DDIs) between antiretrovirals and the estrogen formulations most commonly used by TW. This R21 study is based on the scientific premise that current gaps in the research literature limit the effective utilization of PrEP among TW, and that a better understanding of PrEP uptake, adherence and TFV levels in TW is critical to the development of optimal HIV prevention programs. The project leverages an existing cohort of TW on PrEP at an LGBT-focused community-based health center, in order to better understand and improve PrEP implementation efforts with this population. The specific aims of this R21 project are to: (1) Identify factors associated with PrEP initiation and persistence among a sample of TW with access to health care; (2) Identify correlates, motivators, and predictors associated with objective (i.e., TFV-DP levels) and self-report measures of PrEP adherence among a cohort of TW taking PrEP for at least three months; and (3) Examine the association between self-reported adherence, TFV-DP concentrations, and estradiol levels among TW on PrEP over a 6-month period. To accomplish these aims, we will conduct two complementary exploratory studies. First, we will conduct a detailed retrospective case/control chart review study, comparing 160 TW women who chose to initiate PrEP to 160 yoked TW who refused PrEP in the same 12-month period. Second, we will conduct a prospective cohort study, in which we will follow two cohorts of patients for 6- months: 1) TW who have been on PrEP for at least 3 months (n = 100); and 2) TW who have had access to PrEP services for at least 3 months but have refused PrEP (n = 50). Data collection will include medical record review, HIV/STI testing, self-reported and objective measures of PrEP adherence (i.e., measurement of TFV- DP concentrations), measurement of hormone levels and mode of administration (e.g., patch, tabs, injectable), and self-report survey data on psychosocial variables at enrollment, 3-month, and 6-month follow-up. In addition, we will enroll a subsample of TW using injectable estradiol to examine TFV-DP and hormone concentrations at trough (i.e., injection visit) and peak (i.e., 4-days post injection) levels to identify any potential fluctuations. This study would be the first to explore PrEP initiation, persistence, and adherence among a large cohort of TW in a real world setting and is poised to have an immediate, significant and sustained impact by identifying critical factors necessary to intervention to improve PrEP implementation and reduce HIV incidence.